Brazil’s home region has two peculiarities: first, it is essentially fuzzy as its extension and membership have changed overtime; second, regardless of its limits, its inner core has been characterized by a long period of interstate peace. These factors have led to two outcomes: first, high politics has been conducted through diplomatic rather than military means; second, region-building has remained under the strictest control of the governments rather than becoming self-sustaining. Regional public goods have been mostly defined on the negative, especially as the avoidance of negative externalities, and only recently has Brazil started to invest in the creation of a governance framework that keep extra-regional powers away. Yet, structural limitations and instrumental constraints have limited Brazilian efforts and turned South America into a still peaceful but increasingly divergent sub-region. Through an analysis of institutional overlap and policy networks, especially regarding nuclear energy and the environment, this paper shows that Brazil’s low, late and soft investment in regional security governance is explained by a combination of low regional risks, scarce domestic resources, a legalistic culture of dispute settlement, and transgovernmental networks that substitute for intense interstate cooperation and deep regional institutions.